Field of the Invention
The present invention relates to a method for manufacturing a photomask for use in a manufacturing process of a semiconductor device such as a large scale integrated circuit device (LSI).
A fine processing technique is indispensable in manufacturing a highly integrated semiconductor device such as an LSI. A technique for transferring a pattern to a semiconductor substrate is important in the fine processing techniques. Above all, it is important to transfer a pattern to a resist applied to the semiconductor substrate by means of a exposure technique. In the photolithography which plays an important role in the fine processing techniques, a photomask is the basis from which a fine pattern is transferred to the resist. A fine pattern of the photomask is formed on a transparent glass substrate through a substance which intercepts an ultraviolet light.
FIGS. lA to lD are partially sectional views showing a conventional method for manufacturing a photomask in order of steps disclosed, for example, in Japanese Patent Laying-Open Gazette No. 173251/1986. The conventional method for manufacturing the photomask is described with reference to FIGS. lA to lD.
Referring to FIG. lA, a metal silicide film 2 having the thickness of approximately 1000 .ANG. is formed on the overall surface of a transparent glass substrate 1 such as a quartz substrate by means of a sputtering or an electron beam deposition using a metal silicide such as molybdenum silicide (MoSi.sub.x) or tungsten silicide (WSi.sub.x) as a target.
Referring to FIG. lB, a resist such as PMMA (polymethylmethacrylate) is applied to the metal silicide film 2 in the thickness of 4000 .ANG. to 6000 .ANG. to form a resist film 3. A desired pattern is drawn thereon by ultraviolet lights or electron beams. Thereafter, the resist film 3 having the pattern is developed and baked at temperatures of 100.degree. C. to 140.degree. C.
As shown in FIG. lC, the metal silicide film 2 is etched using the resist film 3 as a mask.
As shown in FIG. lD, the resist film 3 is removed so that a photomask pattern is formed in the metal silicide film 2. The photomask thus formed is utilized for manufacturing a semiconductor device.
According to the above-described conventional method for manufacturing a photomask, the step of etching shown in FIG. lC is that of dry etching, which is performed by a reactive ion etching as a kind of plasma etching using a plane parallel plate type of apparatus. FIG. 2 is a schematic diagram showing an example of apparatus used in the reactive ion etching. Referring to FIG. 2, an anode plate 5a and a cathode plate 5b are provided as plane parallel plates in a chamber 4. The transparent glass substrate 1 on which the metal silicide film 2 and the resist film 3 are formed is placed on the cathode plate 5b. The cathode plate 5b is grounded through an RF power supply 6 having a high frequency.
The step of etching the metal silicide film 2 in a plasma by using the apparatus is described hereinafter. FIGS. 3A and 3B show the process of the plasma etching in accordance with the conventional method for manufacturing the photomask. Referring to FIGS. 2, 3A, and 3B, a sample is placed on the cathode plate 5b. A gas in the chamber 4 is discharged therefrom in the direction shown by the arrow A shown in FIG. 2 so as to keep the atmosphere in the chamber 4 less than 0.1 Torr. Thereafter, a mixed gas of 95%-CF.sub.4 and 5%-O.sub.2 is introduced from a gas bomb 7a into the chamber 4 through a regulator 8 and a valve 9. A mass-flow controller 10 keeps the pressure in the chamber 4 constant, for example, approximately 0.2 Torr. An RF discharge occurs in the chamber 4 at the frequency of 13.56 MHz and the output of 300 W to generate a plasma between the anode plate 5a and the cathode plate 5b. The plasma contains a radical substance which is chemically active. Fluorine radical substance F* is contained in the plasma in this case. The radical substance acts as an etchant for etching the metal silicide film 2. While the metal silicide film 2 is thus etched, a light having a certain wavelength is monitored through a glass 12 and a light receiving portion 11 provided outside the chamber 4 so that the end point of etching is detected by an end point detection unit 13. The switch of the RF power supply 6 is turned off when the end point of the etching is detected. Thereafter, a gas remaining in the chamber 4 is removed therefrom so that the pressure therein is below 0.1 Torr. Nitrogen gas is, then, introduced into the chamber 4 in the direction shown by the arrow B in FIG. 2. As a result, the chamber 4 is maintained at atmospheric pressure. The sample is taken out of the chamber 4. Thus, the metal silicide film 2 formed on the transparent glass substrate 1 is etched.
As described above, a pattern is drawn by the electron beam on the electron beam sensitive resist applied in the thickness of 4000 .ANG. to 6000 .ANG. to the molybdenum silicide film 2. As the sheet resistance of the molybdenum silicide 2 is approximately 100.OMEGA./.quadrature., the molybdenum silicide 2 does not cause a charge-up phenomenon that the electron beams are deflected by the electric charge which has remained on the surface of the metal silicide film. Further, the dry etching of the molybdenum silicide film can be more easily effected than that of a chromium film. For example, when the dry etchings of the molybdenum silicide film and the chromium film are performed in the above-described condition, the molybdenum silicide film is etched at the rate of approximately 500 .ANG. to 1000 .ANG./min, which is approximately 5 to 10 times that of the chromium film. Furthermore, since the metal silicide principally includes silicon, its adhesiveness to the quartz substrate (including SiO.sub.2, Al.sub.2 O.sub.3, and the like) is desirable, so that the metal silicide is not peeled from the quartz substrate. Thus, a photomask can be reliably manufactured.
A photomask thus manufactured is reliable. However, the conventional method has a disadvantage in that dry etching-resistant resists can be only used. Another disadvantage thereof is that positive type resists having resistances to the dry etchings are low in their sensitivities to electron beams, and, therefore, such resists are used at the sacrifice of the throughout of an electron beam exposure device. Further, there occurs a case in which such a resist is not completely removed from the metal silicide, so that a pattern developed on the metal silicide cannot be preferably reproduced.